Electrolytic etching process for electrolytic capacitors



etching tantalum electrode surfaces.

United States Patent ELECTROLYTIC ETCHING PROCESS FOR ELECTROLYTICCAPACITORS George J. Kahan, Williamstown, Mass, assignor to SpragueElectric Company, North Adams, Mass., a corporation of Massachusetts NoDrawing. Application December 31, 1952, Serial No. 329,150

2 Claims. (Cl. 204-141) This invention relates to a process for etchingelectrolytic type capacitor electrodes and more particularly relates toa process for etching tantalum electrodes prior to oxidizing the surfacethereof in order that the same may form one element in an electrolytictype capacitor.

The use of an etching process for preparing metallic surfaces for use aselectrodes in electrolytic type capacitors has long been well-known inthe capacitor art. Up until the time of the instant invention, however,it has been virtually impossible to successfully etch tantalum by anyknown etching process or solution. Moreover, the known etching processeswhich were attempted to be used on tantalum involved tedious, laborious,multiple steps, rendering the same impractical for most commercialapplications.

It is therefore a primary object of the instant invention to avoidand/or eliminate the above-noted deficiencies and to provide arelatively simple process for A further object of the invention is toprovide such type process which will complete the desired etching in arelatively short time, comparable to like processes for easily etchedmetals. Other distinct objects will become apparent from the descriptionand claims which follow.

The invention may be briefly described as comprising a process in whichthe tantalum is immersed in an aqueous solution of hydrofluoric acidcontaining a slight amount of a film-forming ion which is insuflicientto compensate for the complex forming tendency of the hydrofluoric acid,and then impressing a voltage across the bath with the tantalum as anodeuntil the etching is completed. This process may also be carried outwith pure hydrofluoric acid. While the addition of a film forming anionwill modify the action of the bath, the essential function of the bathis carried by the fluoride ions or complex fluoride ions in solution.

The concentration of hydrofluoric acid in aqueous solution may vary fromlow concentrations to the maximum possible wtihout giving rise tovolatilization of hydrofluoric vapors at room temperature. The preferredconcentration which has been found to be most efiicient in operation,however, is the 48% concentration. As a general rule, the concentrationmay be increased as the ambient temperature is lowered, or conversely,must be reduced as the ambient temperature is increased, due to the factthat the volatility of the acid concentration gives rise to noxious,dangerous fumes.

The film-forming anion may constitute any anion which is insufficient tocompensate for the complex forming action of the hydrofluoric acid, theamount of anion used decreasing in relationship to the greaterfilm-forming action inherent in it. Suitable anions are solublechloride, soluble sulfates, soluble nitrates or bromides. The amount offilm-forming anion used will vary according to the strength of itsfilm-forming action up to a concentration of approximately 20% in themanner described.

2,775,553 Patented Dec. 25, 1956 A preferred etching solution contains a38.4% concentration of hydrofluoric acid in an aqueous solution with a7% concentration of hydrochloric acid. Preferably this electrolyte isused with low ambient temperatures varying between 40 and 50 F.,although it may be carried out at higher temperatures with suitableprovision for the removal of any noxious fumes given off by theelectrolyte bath.

The following is listed as one example of the invention.

Example A tantalum electrode surface is immersed in an electrolyticetching bath comprising an aqueous solution containing 38.4%hydrofluoric acid and 7% hydrochloric acid. A potential of 8-15 volts isthen impressed across the electrolyte bath and tantalum electrode withthe tantalum as the anode (the current density being maintained to be atleast 2 amps. per square inch or higher) and the etching allowed toproceed for a period of 1 to 2 minutes. After removal from the bath theexposed surface of the electrode is shown by inspection to have asatisfactory etch ratio. After etching the tantalum electrode iscleansed, and then oxidized to produce the desired oxide film on theetched surface. Etch ratios of 3 to 5 resulted. The completed electrodeis then combined with a suitable electrolyte in usual manner ofelectrolytic capacitors.

The particular action by which the etching proceeds is not at presentwholly understood but is definitely indicated to be critically limitedby the amount of film-forming anion in the aqueous hydrofluoric acidsolution, the critical limit being reached when the additive becomessufiicient to compensate for the complex forming action of thehydrofluoric acid.

It has been further found that the electrolytic etching means is notrestricted to direct current but that an alternating potential isequally satisfactory. The use of the alternating potential is notlimited to the etching of tantalum but is highly satisfactory also foraluminum.

The great advantages of this discovery is quite apparent in that nolonger will rectification of the electrical poten-. tial be necessary.

As many apparently widely different embodiments of this invention may bemade Without departing from the spirit and scope hereof, it is to beunderstood that the of from 38.4 to about 48% hydrofluoric acid and upto u about 20% .of an anion selected from the group consisting ofchloride, bromide, nitrate and sulfate.

2. The process of claim 1 in which the aqueous solution contains 38.4%hydrofluoric acid and 7% hydrochloric acid.

References Cited in the file of this patent UNITED STATES PATENTS2,481,306 Gall et a1. Sept. 6, 1949 FOREIGN PATENTS 512,764 Belgium July31, 1952 OTHER REFERENCES Treatise on Inorganic Chemistry, by Mellor,vol. 9 (published in 1929), page 891.

1. A PROCESS FOR INCREASING THE EFFECTIVE SURFACE OF METALLIC TANTALUMCOMPRISING ANODICALLY TREATING IT AT AN ANODIC CURRENT DENSITY OF ATLEAST 2 AMPERES PER SQUARE INCH IN AN AQUEOUS SOLUTION CONSISINGESSENTIALLY OF FROM 38.4 TO ABOUT 48% HYDROFLUORIC ACID AND UP TO ABOUT20% OF AN ANION SELECTED FROM THE GROUP CONSIST ING OF CHLORIDE,BROMIDE, NITRATE AND SULFATE.